In general, the invention relates to how in a cellular radio system there is made a decision that the mobile station is handed over to a new base station. In particular, the invention relates to how the different base stations are set in a priority order for the handover.
In cellular radio systems, there is known a so-called handover procedure, according to which a data transmission connection between a mobile station and the stationary parts of the system is routed to pass via a new base station, when the connection through the old base station becomes too weak or has too much interference. For instance in a GSM system (Global System for Mobile telecommunications), each base station transmits a signal in a given so-called BCCH channel (Broadcast Control Channel), in which case the mobile stations measure the power of the received BCCH signals and determine on the basis thereof which cell is the most profitable for the quality of the radio connection. The base stations also inform the mobile stations of the BCCH frequencies used in the adjacent cells, so that the mobile station know what frequencies they must listen to in order to find the BCCH transmissions of the adjacent cells.
FIG. 1 illustrates a second-generation cellular system comprising a mobile switching centre MSC belonging to the core network CN of the cellular system, as well as base station controllers BSC and base stations BS belonging to a radio access network RAN; the mobile stations MS are in connection with said base stations. FIG. 2 illustrates the coverage areas 201a-209a of the base stations 201-209 in another second-generation cellular system.
In second-generation cellular systems, such as the GSM system, data transmission between the base stations BS and the core network CN takes place through base station controllers BSC. One base station controller normally controls a large number of base stations, so that when a mobile station moves from the area of one cell to the area of another, the base stations of both the old and the new cell are very often connected to the same base station controller. Thus the selection of a new active base station can be carried out in the base station controller. Consequently, for example in a regular GSM system there occur fairly few such inter-cell handovers where the mobile station moves from a base station connected to a first base station controller to a base station connected to a second base station controller. If this should happen, the switching centre must close the connection with the first base station controller and set up a new connection with a new base station controller. This kind of procedure includes a lot of signalling between the base station controllers and the switching centre, and because the distances between the base station controllers and the switching centre may be long, interference may occur during the selection of a new base station and a new base station controller.
A prior art arrangement for changing the active base station and base station controller is well suited to so-called second-generation digital cellular radio systems, such as GSM and its expanded version DCS1800 (Digital Communications System at 1800 MHz), IS-54 (Interim Standard 54) and PDC (Personal Digital Cellular). However, it has been suggested that in the future third-generation digital cellular radio systems, the quality of service offered by the cells for the mobile stations may considerably vary from cell to cell. Suggestions for third-generation systems are UMTS (Universal Mobile Telecommunications System) and. FPLMTS/IMT-2000 (Future Public Land Mobile Telecommunications System/International Mobile Telecommunications at 2000 MHz). In these plans, cells are divided, on the basis of their size and characteristics, for instance to pico, nano, micro and macro cells, and for example data transmission capacity can be used to describe the quality of service. The highest data transmission capacity is offered by pico cells and to the lowest in macro cells. The cells may be partly or completely superimposed, and there may be different kinds of mobile terminal devices, in which case all mobile stations cannot necessarily make use of the quality of service offered by all cells. Moreover, base stations can in different ways support services requiring real-time and non-real-time data transmission.
FIG. 3 illustrates a form of a future cellular radio system, which is not totally new for instance with respect to the known GSM system, but contains both known elements and completely new elements. The bottleneck of current cellular radio systems that hinders the offering of more advanced services to the mobile stations, is the radio access network RAN formed by the base stations and the base station controllers. The core network of the cellular radio system consists of mobile services switching centres (MSC), other network elements (in GSM for instance SGSN and GGSN connected to packet radio transmission, i.e. Serving GPRS Support Node and Gateway GPRS Support Node, where GPRS means General Packet Radio Service), and of transmission systems connected thereto. The core network is capable, in accordance with GSM+ definitions developed from GSM, of transmitting new types of services, too.
In FIG. 3, the core network of the cellular radio system 300 is a GSM+ core network 301, and three parallel radio access networks are connected thereto. Among these, networks 302 and 303 are UMTS radio access networks, and network 304 is a GSM+ radio access network. Of the UMTS radio access networks, the one illustrated topmost, i.e. 302, is for example a commercial radio access network owned by a teleoperator that offers mobile communications services and serves equally all customers who are subscribers of said operator. The lower UMTS radio access network 303 can be private, owned for instance by an enterprise in whose facilities said radio access network functions. The cells in the private radio access network 303 are typically nano and/or pico cells, and only the terminals of the employees of the owner enterprise can camp in them. All three radio access networks can contain cells that offer different types of services and vary in size. Moreover, the cells of all three radio access networks 302, 303 and 304 can be completely or partly superimposed. The bit rate applied in each case depends among others on the radio environment, features of the employed services, the regional total capacity of the cellular radio system and the capacity needs of other users. The above mentioned new types of radio access networks are in general called generic radio access networks (GRANs). Such a network can be connected to be used in cooperation with different types of core networks CN, and particularly with the GPRS network of the GSM system. A generic radio access network GRAN can be defined as a group of such base stations BS and radio network controllers RNC controlling them where the members of the group are capable of exchanging signalling messages. In the specification below, the generic radio access network is called radio network GRAN for short.
The mobile station 305 illustrated in FIG. 3 is most advantageously a so-called dual mode station that can function either as a second-generation GSM terminal or a third-generation UMTS terminal, according to what kind of services there are available in the area where it is located at the point of time in question and what are the data transmission needs of the user. It can also serve as a multi-mode terminal that can function as the mobile station of several different data transmission systems according to the needs and availability of services. The radio access networks and services available for the user are defined in the subscriber identity module SIM 306.
FIG. 4 illustrates in more detail the core network CN of a third-generation cellular radio system, the CN comprising a switching centre MSC, and the radio network GRAN connected to the core network. The radio network GRAN comprises radio network controllers RNC and base stations BS connected thereto. Now a given radio network controller RNC and the connected base stations are capable of offering services at a wide frequency band, and another radio network controller and connected base station may be capable of offering only traditional narrow-frequency services, but possibly with a larger coverage area.
FIG. 5 illustrates the coverage areas 501a-506a of the base stations 501-506 in a third-generation cellular radio system. As is observed in FIG. 5, a stationary terminal or even one that moves for a short length can select among several different base stations when setting up a radio connection.
Let us now investigate how a prior art arrangement is applied in the designed third-generation digital cellular radio system. In third-generation systems, changes of the active base station and the active radio network controller are remarkably common in comparison with second-generation systems. This is, among others, due to the fact that the cell sizes may be extremely small, and that during the radio connection, the type of service is wished to be changed for example from narrow-band to wide-band. Also the beginning or ending of various services requiring real-time or non-real-time data transmission can affect the need to change the base station or the base station controller.
A prior art measurement of the power level of the received signal in the mobile station does not give the best possible impression as to how the new base station is capable of responding to the service needs of the mobile station. If the handover of the base station and/or the radio network controller is often executed so that the new routing for the connection is not, after all, the best possible in relation to the service needs of the mobile station, the network is loaded due to unnecessary handovers. The switching centre should perform an extremely large number of connection cut-offs/setups, which requires a high amount of extra signalling between the switching centre and the radio network controllers. Moreover, in the area of one switching centre, there is a remarkably high number of small-size cells, and in wide-band applications, the quantity of user data to be transmitted also is extensive. This results in extremely high capacity and speed requirements for the switching centre equipment, which cannot in large systems be realised at reasonable costs when using the current technology.
The object of the present invention is to introduce a method and an arrangement for executing handover so that the possibilities of the base stations to offer services needed by the mobile station are taken into account.
The objects of the invention are achieved by putting the potential new base stations in priority order on the basis of the level of the carrier to interference ratio they can offer, as well as to the existing load of the base stations.
The method according to the invention is characterised in that in order to select the new base station, there is chosen a group of potential new base stations and estimated a carrier to interference ratio that each of the potential new base stations belonging to said group could offer.
The invention also relates to a cellular radio system that is characterised in that it comprises, for choosing a new base station, means for estimating the carrier to interference ratio that each of the base stations belonging to the group of potential new base stations could offer.
According to the invention, an estimate is made of each potential new base station; this estimate is based on the measured signal loss between the base station and the mobile station, on the knowledge of the total transmission power used by the base station and the interference level prevailing in the base station cell area, as well as on the knowledge of the free data transmission capacity available in the base station. These estimates are maintained continuously, and the selected new base station will be the one that is the most advantageous according to the estimate. In order to prevent a situation where the mobile station remains ping-ponging back and forth between two such base stations that have fairly similar advantages according to the estimates, there can be required a hysteresis where the mobile station hands over to a new base station only if the estimate of the new base station surpasses the estimate of the current base station for a given threshold value.
The signal loss between the base station and the mobile station is calculated by studying the difference between the transmission power of the base station on a given control or pilot channel and the respective received power at the mobile station. All base stations can use the same transmission power on the control or pilot channel, or each base station can inform the mobile station as to what power is used in any given case. In order to define the level of total transmission power and interference power, each base station calculates a given total power parameter and a given interference power parameter, and on the basis of these it is predicted how high the achieved C/I ratio (carrier to interference ratio) would be between the base station and the mobile station both in the uplink and downlink directions. In addition to this, there are calculated load factors for potential new base stations with respect to both real-time and non-real-time data transmission. In a preferred embodiment of the invention, the load parameter connected to real-time data transmission indicates whether the data transmission capacity available at the new base station is as large as the one reserved for the mobile station at the current base station, and the load parameter connected to non-real-time data transmission indicates how large a share of the data transmission capacity of the new base station allocated for non-real-time data transmission is available.
An estimate describing the advantages of the new base station is compiled of the predicted C/I ratios and load parameters, which all are preferably calculated separately for the uplink and downlink connections. The uplink and downlink directions can, when forming the estimate, be weighted differently, if it is wished to emphasizes the significance of one or the other for the handover. According to the invention, connections requiring real-time data transmission are oriented towards cells where the best C/I ratio is obtained, and connections requiring non-real-time data transmission are oriented towards cells with the best channelwise weighted C/I ratio, i.e. the largest quantity of xe2x80x9cfreexe2x80x9d transmission energy. From the point of view of the operation of the system, the invention attempts to maximise the C/I ratio in each connection and at the same time to minimise the total transmission power to be used, so that the available resources are optimally utilised.